DETERMINATION OF ELECTROLYTES AND BLOOD GASES. unilag mbbs/bds 3oo level, 2nd semester.

Collection of specimens:
Blood is collected into heparinised glass or plastic syringes. or capillary tubes.. The blood is injected directly into the analyse and electrolytes and blood gases are analysed simultaneously.

Preparation of patient:
The patient must be in a steady state, breathing normally.If there is anxiety or exercise, let the patient rest for about 15 minutes.
Patients on respirator or on oxygen supplement must be stabilised by keeping the regulator constant for at least 30 minutes before sampling.

Anxiety of venepuncture increases respiration; it may therefore be necessary to control the anxiety by giving local anaesthesia and inserting an indwelling catheter if multiple sampling will be required. If a catheter is used, withdraw 3 times the dead space of the catheter before sampling.

Container:
Glass syringe or capillary is preferred to plastic: It is required that the container gas tight for be at least two hours. Glass is inert and impermeable to gases while plastic is permeable; the permeability varying with tire thickness of the wall of the syringe, the surface to volume relationship and the tightness of the stopper.

The rate of escape of the gases depends on the gradient between the blood and the atmospheric values. The po2 value of gg mm Hg (r2kpa) increases uy t to 9 mm Hg (0’13-1’20 kPa) after 45 minutes, most of the changes occurring in the first 20 minutes.

PCo2 changes are insignificant in the first 30 minutes if stored in ice.In general, changes are minimised by transporting the specimen in ice until analysed

Therefore, blood in plastic well-stoppered, anticoagulated syringe, transported in ice water slush is most suitable for the determination of pH, pco2, bicarbonate,
base excess and the electrolytes.

If determination of po is also required from the same specimen, the analysis must be carried out within 15 minutes, otherwise a glass syringe should be used for PO2 estimation.

Anticoagulant:
sodium or lithium heparin, pH 6-8 is used. as suitable. It should be pure and free from ions that may bind to calcium e.g bicarbonate, surphate, phosphate and formate etc.

Sodium heparin in <15 I.U. per ml blood does not affect the sodium concentration.
Heparin, sodium or lithium lowers pH, bicarbonate, base excess and calcium when used in concentration of up to 100 I.U per ml blood

pH = decreases by 0 to 0.004,

Bicarbonate = 0 to 0.3mmol/L,

Base excess = 0 to 0.3mmol/L,

Calcium = about 0.13 mmol/L

The heparin concentration should be kept low.

Glass activates the coagulation factors and platelets more rapidly than plastic does; glass therefore requires more heparin.
The following concentrations of heparin have been found suitable:

glass = dry = 10 to 60 I.U./ml,

glass = solution = 8 to 12 I.U/ml,

Plastic = dry = 12 to 18 I.U/ml,

Plastic = solution = 4 to  6 I.U/ml

To keep changes minimal, the dead space of the syringe should not exceed 5 percent of the blood taken. At 5 percent

. pH does not change
. PCO2 decreases by 2.0 mmHg (0.27 kPa)
. Bicarbonate and base excess each decreases by 1.2 mmol/L
. PO2 increases by 4 mm Hg (0.53 kPa)

The heparin concentration recommended do not significantly affect the Sodium,
Potassium and Chloride values within the normal range and calcium concentration
in the 1.2-1.4 mmol/L range.

Specimen collection:

The sample must be collected anaerobically and anticoagulated to prevent clot
formation.. Avoid haemolysis; this increase the potassium content and decreases
the calcium and pH.

Arterial blood

The radial, brachial, femoral, scalp (children), umbilical (neonates) arteries
are used. The artery is canulated for repeated sampling.
The specimen is suitable for the determination of:

1. Lung function
2. Electrolyte and acid – base

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